1. Field of the Invention
The invention relates to a device for correcting a video signal affected by line tilt, including an adder circuit for adding a correction signal to a video signal to be corrected, said video signal to be corrected being conveyed with its total passband to a first input of the adder and the corrected signal being available at an output of the adder.
Line tilt is a signal deformation caused by the overall transmission-transfer-receiving assembly, and more specifically by the poor tuning control of the receivers, a frequency shift introducing a deformation because of the attenuated side band.
On reception, the effect shows on the white or grey levels within a video line, in the form of inclined plateaus.
This effect is particularly annoying in the case of a scrambled transmission by pseudo-random permutation of line sections, as, when introduced in the transfer link between the scrambler and the descrambler, the line tilt becomes evident in the fact that the picture elements which are added during the recording operation on either side of the cut off point do not have the same luminance level. The descrambling operation converts this distortion in a "line noise" which is distributed over the image by the pseudo-random generator.
A device to correct this fault is therefore used mainly in television receivers, but can alternatively be useful in professional applications for example for the correction before transmission in cable network head ends.
2. Description of Related Art
The document "A video scrambler/descrambler concept for the PAL format" by M. CHRISTIANSEN et al, Journal of the Institution of Electronic and Radio Engineers, vol. 57, no. 1, pp. 27-35 Jan./Feb. 1987, describes the correction of the line tilt by adding to the video signal a sawtooth of the line frequency, whose amplitude and sign are adjusted to provide the best result.
It has nevertheless been found that by simply adding a sawtooth, it is not possible to correct all the signal types, more specifically because the slope of the level depends on their amplitude. If instead of adding a signal, the signal is multiplied by a sawtooth, it will be found that the method only gives good results in particular cases.